The proposed research is designed to obtain data and test hypotheses that will increase our understanding of the function of molybdenum in redox enzymes such as xanthine oxidase, sulfite oxidase, nitrate reductase and nitrogenase, by a study of model chemical systems based on molybdenum complexes. The structures, esr and electronic spectra, redox potentials and magnetic properties of Mo(VI), (V), (IV), (III) complexes with ligands such as cysteine and related thiols, cysteine containing peptides and small proteins, other amino acids and flavins will be determined. The kinetics and mechanisms of the reactions of these complexes with substrates and model compounds such as nitrate, sulfite, purines, aldehydes, nitrogen, flavins, iron complexes, ferredoxins and cytochromes will be investigated as models for the enzymatic reactions. The results will be used to interpret the corresponding properties of the molybdenum enzymes and to improve our understanding of the mechanisms of metal-enzyme catalyzed redox reactions.